断点调试Autolayout

对于 iOS 和 OS X 开发者来说，Autolayout已经逐渐变成一个至关重要的开发工具。它让多屏幕适配变得小菜一碟(peasy)，但是有些时候它还是会把我们搞疯掉，因为它总是会出现那些啰嗦又没啥用处的错误警告。

就像这样:

Unable to simultaneously satisfy constraints.
Probably at least one of the constraints in the following list is one you don't want.
Try this:

(1) look at each constraint and try to figure out which you don't expect;
(2) find the code that added the unwanted constraint or constraints and fix it.
(Note: If you're seeing NSAutoresizingMaskLayoutConstraints that you don't understand, refer to the documentation for the UIView property translatesAutoresizingMaskIntoConstraints)

(...........)


Make a symbolic breakpoint at UIViewAlertForUnsatisfiableConstraints to catch this in the debugger.
The methods in the UIConstraintBasedLayoutDebugging category on UIView listed in  may also be helpful.

这么长的错误日志！！！！尼玛让谁看！！！
但是我们仔细看一下，观察 NSLayoutConstraint 部分。发现它倒数第二行还是有给我们点希望去解决这个错误的。在 UIViewAlertForUnsatisfiableConstraints添加一个Symbolic breakpoint 断点。
既然这样，我们就去试一下...

尼玛！！ 毛用都没有啊！！它停在了线程堆栈上，然而LLDB依旧一片黑暗...

于是这里就有一个小技巧(Trick)，来让你的symbolic breakpoint变得更加有用。为你的ObjC项目添加 po [[UIWindow keyWindow] _autolayoutTrace]

或者为你的Swift项目添加 expr -l objc++ -O -- [[UIWindow keyWindow] _autolayoutTrace]

现在你就会在命令控制台看到你的UIView 的层次结构还有出现错误的地方。

UIWindow:0x7f9481c93360
|   •UIView:0x7f9481c9d680
|   |   *UIView:0x7f9481c9d990- AMBIGUOUS LAYOUT for UIView:0x7f9481c9d990.minX{id: 13}, UIView:0x7f9481c9d990.minY{id: 16}
|   |   *_UILayoutGuide:0x7f9481c9e160- AMBIGUOUS LAYOUT for _UILayoutGuide:0x7f9481c9e160.minY{id: 17}
|   |   *_UILayoutGuide:0x7f9481c9ebb0- AMBIGUOUS LAYOUT for _UILayoutGuide:0x7f9481c9ebb0.minY{id: 27}

而当你在这个地方继续向下执行，它就会停在下一个你可能出现错误的地方。但是如果这样子你还是很难发现你的错误的话，你可以执行下面这个语句

(lldb) e id $myView = (id) 0x7f9ea3d43410
(lldb) e (void)[$myView setBackgroundColor:[UIColor blueColor]]

先获取UIView，然后改变它的背景色

你就会看到出错误的视图主动显示出来了~

ps: 如果是手写Autolayout，推荐 Mansory，实在是好用！！

关键是它的错误提示

Unable to simultaneously satisfy constraints......blah blah blah....
(
    "",
    "= 5000>",
    "",
    ""
)

Will attempt to recover by breaking constraint
= 5000>

已经把出现错误的约束显示出来了，完全不需要我们去茫茫去偶遇~